1. Field of the Invention
The present invention generally relates to inhibition of scale in industrial process streams. More particularly, the present invention relates to inhibiting scale (e.g., reducing or preventing the nucleation and/or growth of mineral scale solids) produced during operation of wet process phosphoric acid (WPA) production streams.
2. Description and Current Status of the Related Art
About 90% of the world's phosphoric acid is produced according to the wet process, which is conventionally prepared by acidulating phosphate rock (which contains calcium phosphate) with sulfuric acid to yield a crude wet-process phosphoric acid (WPA) and insoluble calcium sulfate (gypsum).
The manufacture of phosphoric acid is well known and is the subject of numerous text books. An overall view of the manufacture of phosphates and phosphoric acid is treated by Becker in Phosphates and Phosphoric Acids, Marcel Dekker, Inc. 1989; and by Slack in Phosphoric Acid, Part 1 and Part 2, Marcel Dekker, Inc. 1968. In the process, calcium phosphate rocks are cleaned in the wash plant and ground in the Ball mill before being fed into a series of reactors for digestion with sulfuric acid along with recycled phosphoric acid from the process. After digestion, the reaction slurry is filtered to separate phosphoric acid from gypsum. The filtered, crude WPA is then sent to clarifiers and evaporators for further purification and concentration. The purified phosphoric acid is either sent out as Merchant Grade Acid (MGA) or continued to make 69% P2O5 Super Phosphoric Acid (SPA), where it can be converted to many end products ranging from a chemical reagent, rust inhibitor, food additive, dental and orthopaedic etchant, electrolyte, flux, dispersing agent, industrial etchant, fertilizer feedstock, and component of home cleaning products. For example, crude phosphoric acid is concentrated to 54% (P2O5) before sent for Monoammonium Phosphate (MAP), Diammonium Phosphate (DAP), or ammonium phosphate-sulfate (APS) production.
As noted in U.S. Pat. No. 5,080,801 to Molter et al. (1992), which teaches various mixed polymers for preventing scale in mineral process waters from a variety of processes, due to the highly acidic environment which is inherent to phosphoric acid production, these plants experience scaling problems unique to this industry. Accordingly, solutions that may be useful for reducing or preventing scale in some industrial processes may not prove suitable for use in the phosphoric acid production stream. U.S. Pat. No. 5,456,767 to Shah et al. (1995) describes a similar sentiment with regard to the use of corrosion inhibitors in refinery overheads (noting that because the refinery overhead environment is extremely acidic, the corrosion inhibitors generally used in other oil field environments are not generally suitable for use with the refinery overheads).
Crude WPA contains significant amounts of dissolved impurities including carbonaceous matter, silica, and many metallic contaminants. Due to the supersaturated nature of the acid and the impurities in the phosphate ores, the concentration steps with respect to P2O5 render several side reactions, causing scale formation and/or deposition in and/or on the equipment in contact with the WPA at different stages of the phosphoric acid production process. For example, scale from the phosphoric acid production process forms on filter cloth and pipes, heat exchangers, evaporators, concentrators, valves, and pipes during the repetitive flashing/cooling/concentrating process of the phosphoric acid production process. Twelve to fifteen different types of scaling species can usually be found throughout the phosphoric acid production process and they pose significant challenges for the industry. Moreover, different phosphoric acid production plants experience different types of scale. Even within one plant, the type of scale can differ greatly between steps in the process or even between phosphate ore composition. Plants normally have to shut down production every few weeks to physically remove the scale using high-pressure water and/or mechanical means. Valuable operating time is lost during this descaling phase resulting in reduced process capacity and ultimately reduced profits.
While some proposed solutions have focused on physical means to remove scale formation and/or deposition on equipment surfaces in the phosphoric acid production process, most have tried to solve the problem by developing a chemical-based reagent. This is the preferred approach because it requires a limited amount of capital investment and does not alter the existing process in the phosphoric acid plants. It also does not require a large amount of reagent and is therefore considered both environmental, and to have a minimal downstream impact. However, due to the complexity of the scale forming issues (e.g., processes of nucleation, crystal growth, and deposition), it is a great challenge to develop reagents useful for inhibition of scale formation and/or deposition on surfaces in contact with digested phosphate rock.
For example, U.S. Pat. No. 5,120,519 (1992) discloses methods of preventing sodium fluorosilicate containing scales on surfaces in contact with digested phosphate rock such as in phosphoric acid production using certain anionically charged vinyl addition water soluble polymers containing from 5 mole % to 100 mole % of a an anionic vinyl monomer, and having a molecular weight of at least 1,000,000.
Chinese Patent No. 1762857 (2006) discloses a scale inhibitor agent for use in wet process phosphoric acid production, which contains mixtures of certain phosphoric or phosphonic acids as scale inhibitors with certain polymers or copolymers as scale dispersants, and a sterilizer compound.
Chinese Patent No. 1724965 (2006) discloses methods for descaling phosphoric acid concentration heat exchangers by employing a washing liquid containing a 5-20% fluosilicic acid solution, a phosphonic acid such as amino trimethylene phosphonic acid, hydroxyl ethylidene diphosphonic acid, and hydroxyl-1,1-ethylidene diphosphonic acid, and a film forming matter that includes nitrous phenylhydroxylamine ammonium salt, urotropine, and sodium molybdate.
Several patents to Harper et al. (U.S. Pat. No. 3,972,981 (1976) and U.S. Pat. No. 4,221,769 (1980); and GB Patent Nos. 1,406,884 (1975) and 1,433,123 (1976)) disclose processes for reducing the deposition of gypsum or calcium sulphate scale during the washing of the hemihydrate filter cake in a wet process phosphoric acid production stream by bringing the anhydrite or hemihydrate crystals into contact with various surface-active agents after the crystals have been formed, but before or during the washing thereof, thereby indicating that the point of incorporation of the additive affects the extent of scale inhibition achieved.
Several patent applications assigned to Cytec Industries Inc. have also addressed scale inhibition in wet process phosphoric acid production by using various chemical reagents. In U.S. Published Application No. 2011/0076218, methods are disclosed for inhibiting scale by adding a reagent having an aliphatic or aromatic compound having two or more hydroxyl groups, and at least one amine. Other suitable reagents disclosed include polyethyleneimine, or derivatives thereof, and other polyamines.
In U.S. Published Application Nos. 2011/0076219 and 2012/0244058 methods for preventing or reducing scale in wet process phosphoric acid production processes using various water-soluble functional organic reagents including certain polymers and/or copolymers are disclosed. This work has resulted in PHOSFLOW® Scale Inhibitor (commercially available from Cytec Industries Inc., Woodland Park, N.J.).
The economic impact for the scale-related issues is substantial, and the industry is in need of a more efficient scale prevention technology than the existing physical means of post-scale formation removal. However, while the various chemical-based reagents discussed above may have some merits and applicability in wet process phosphoric acid production, they are not currently in widespread use. Accordingly, the compositions and methods presently available for inhibiting the formation and/or deposition of scale in the phosphoric acid production process require further improvement. Compositions and formulations that effectively prevent and/or reduce (i.e., inhibit) formation and/or deposition of scale on equipment surfaces in contact with scale-forming ions released from digested phosphate rock, thereby enabling the phosphoric acid production plant to run longer without shutting down to physically remove scale, would be a useful advance in the art and could find rapid acceptance in the industry.